Autodeposited coatings with increased surface slip

ABSTRACT

In a process wherein a resinous coating is formed on a metallic surface by immersing the surface in an acidic aqueous coating composition containing dispersed particles of resin, wherein said composition forms on said surface a resinous coating which increases in thickness the longer the surface is immersed in the composition and wherein the resinous coating is contacted with an aqueous solution containing Cr in order to improve properties (e.g. corrosion resistance), the surface slip of the applied coating is increased by having dispersed in the Cr-containing solution a polymer which has a lower coefficient of friction than the applied resinous coating.

This is a continuation of pending application Ser. No. 66,858, filedAug. 15, 1979, now abandoned which is a division of application Ser. No.917,560, filed June 21, 1978, now U.S. Pat. No. 4,186,266.

BACKGROUND OF THE INVENTION

A relatively recent development in the coating field is the provision ofwater based coating compositions which are effective, without the aid ofexternally applied electricity, in forming on metallic surfaces immersedtherein organic coatings that increase in thickness or weight the longerthe time the surfaces are immersed in the compositions. (Forconvenience, a coating formed from such a composition is hereafterreferred to as "an organic coating which grows with time" or as an"autodeposited coating".) Speaking generally, compositions which are soeffective comprise acidic aqueous coating solutions having dispersedtherein particles of an organic material such as resin particles.Autodeposited coatings are formed from such compositions as a result oftheir ability to attack the metal surface and to generate metal ions inamounts which cause the particles to deposit on the surface in a mannersuch that there is a continuous buildup of organic coating on thesurface.

Coatings formed from such compositions are distinctly different fromcoatings formed by immersing the metallic surfaces in conventionallatices, that is, compositions comprising resin particles dispersed inwater. The weight or thickness of a coating formed by immersing ametallic surface in a conventional latex is not influenced by the timethe surface is immersed in the latex. It is in the main influenced bythe amount of resin solids dispersed in the aqueous medium.

Coatings formed from the aforementioned recently developed coatingcompositions are also distinctly different from coatings formed fromearlier known acidic aqueous coating solutions containing dispersedsolid resin particles and relatively high amounts of water solublecorrosion inhibitors, such as compounds containing hexavalent chromium.The use of relatively high amounts of corrosion inhibitors in suchsolutions deters attack of the metallic surface to an extent such thatresinous coatings which grow with time are not obtained. Thus, resinouscoatings formed by immersing metallic surfaces in such compositions arelike those formed from immersing the metallic surfaces in conventionallatices in that they do not grow with time.

The use of the recently developed coating compositions which producecoatings which grow with time offers a number of advantages. Forexample, other factors held constant, they can be used to apply thickerorganic coatings to the metallic surface in a shorter period of time andin a one-step operation. Also, the coating thickness can be controlledby varying the immersion time of the metallic surface in the coatingcomposition. In general, coatings which have improved corrosionresistant properties and aesthetic appearance are obtainable. These arebut a few of the advantages which flow from the use of saidcompositions.

While the aqueous acidic coating compositions described above produceexcellent coatings, it is frequently desired to increase the corrosionresistance of such applied coatings (as measured, for example, by ASTMB117).

The commonly employed method of increasing the corrosion resistanceproperties of autodeposited coatings is to contact the wet or unfusedapplied coating with an aqueous Cr-containing solution. Typically, theaqueous Cr-containing solution can be an aqueous solution of hexavalentchromium, the source of which may be a water or acid soluble chromate ordichromate or CrO₃ ; or an aqueous solution of hexavalent chromium andformaldehyde-reduced forms of hexavalent chromium. For example, see U.S.Pat. Nos.: 3,585,084; 3,592,699; 3,647,567; and 3,795,546.

While the corrosion resistance of the applied autodeposited coatings canbe increased by treatment with aqueous Cr-containing solutions, thecorrosion resistance obtained will vary from one autodeposited coatingto another due to the large variety of resins which can be applied bythis method.

In some end use applications it is desired that the resin coatedmetallic surface have a relatively high degree of surface slip. This isthe case in applications wherein the surface of a coated metal part isin movable surface contact with another coated or uncoated metal partand it is desired that the two surfaces slide across one another easilywithout scratching the coating (e.g. a coated metal track for a slidingdoor, trunk hinge, door latch, etc.).

One can achieve surface slip in an autodeposited coating by using anaqueous acidic coating solution containing, as the dispersed resintherein, a resin having an inherently high degree of slip (e.g.polytetrafluoroethylene or polyethylene). Unfortunately, the otherproperties of such autodeposited resins--in particular their corrosionresistance--may not meet the performance criteria for their intendeduse. Autodeposited coatings of polytetrafluoroethylene and polyethylenedisplay relatively low corrosion resistance by comparison with othertypes of autodeposited resins. Polytetrafluoroethylene has anotherdrawback in that very high temperatures are required to fuse the appliedcoating into a continuous film.

It is, therefore, an object of this invention to provide a method ofapplying an autodeposited coating having a high degree of surface slipwhile concomitantly providing a high degree of corrosion resistance inthe applied coating.

SUMMARY OF THE INVENTION

There is provided in accordance with the teachings of this invention, amethod of applying an autodeposited resinous coating which has a highdegree of surface slip with no significant sacrifice in corrosionresistance. In the method of this invention an autodeposited resinouscoating is first applied to a metallic surface in the known manner.While the applied coating is still wet, that is uncured, it is contactedwith an aqueous Cr-containing solution which has dispersed thereinparticles of a resin which is different from the resin which has beenautodeposited on the metallic surface and which has a lower coefficientof friction than the resin which was autodeposited on the metallicsurface.

The coating on the metallic surface which has received the treatmentdescribed in the foregoing paragraph can then be fused into a continuousfilm by heating. While not wishing to be bound by any particular theory,it is believed that some of the resin particles dispersed in the aqueousCr-containing solution are deposited in the surface layers of theautodeposited coating and, upon fusing, become an integral part thereofwhere they are concentrated and to the extent that they are capable ofaltering the surface characteristics thereof.

DETAILED DESCRIPTION OF THE INVENTION

Coating compositions which are effective in forming organic coatingswhich grow with time are known. Examples of such coating compositionsare described in U.S. Pat. Nos. 3,585,084; 3,592,699; 3,709,743; and3,776,848 in British Pat. No. 1,241,991, in South African Pat. No.72/1146 and in Belgian Patent of Addition No. 811,841.

Speaking generally, the acidic aqueous coating compositions of theaforementioned type function to attack the metallic surface contactedtherewith and to generate metal ions in an amount sufficient to directlyor indirectly cause organic particles in the region of the metallicsurface to deposit thereon in a continuous fashion, that is, in a mannersuch that there is a buildup in the amount of organic material depositedon the surface the longer the time the surface is in contact with thecomposition. This deposition of the organic material on the metallicsurface is achieved through chemical action of the coating compositionon the metallic surface. The use of externally applied electricity whichis necessary for the operation of some coating methods, such as theelectrocoating method, is not required.

The present invention is used in connection with coatings formed fromcompositions that contain solid particles of an organic polymerdispersed in the aqueous phase of the composition.

A preferred composition for applying the autodeposited coating in thepractice of the present invention is described in Belgian Pat. No.840,145, issued in the name of Wilbur S. Hall. The preferredcompositions has a pH of about 1.6 to about 4 and is prepared fromwater, a ferric-containing compound, most preferably ferric fluoride, inan amount such that it contains the equivalent of about 0.5 to about 3.5g/l of ferric iron, about 0.2 to about 5 g/l of HF, a pigment such ascarbon black, and about 5 to about 550 g/l of resin particles preferablyfrom 50 to 100 g/l of resin particles, which are all of substantiallythe same size and substantially chemically homogeneous, that is, eachparticle is comprised of the same monomeric constituents present insubstantially the same proportions of resin particles which are preparedby copolymerizing the following monomers:

(1) about 25 to about 70, and preferably about 40 to about 65 wt. % of aconjugated diene having, for example, 4 to about 9 carbon atoms, such asbutadiene or isoprene;

(2) about 5 to about 70, and preferably about 30 to about 65 wt. % ofCH₂ ═CHR, wherein R is an aryl or a cyano group, for example, styrene oracrylonitrile;

(3) about 1 to about 50, and preferably about 3 to about 15 wt. % of avinyl halide such as vinyl chloride or vinylidene chloride; and

(4) about 0.5 to about 15, and preferably about 1 to about 4 wt. % of amonoethylenically unsaturated monomer having a functional group selectedfrom the class consisting of amide and carboxylic groups, such asacrylamide, methacrylamide, octyl acid maleate and monoethylenicallyunsaturated monocarboxylic and dicarboxylic acids having about 3 toabout 12 carbon atoms, and preferably about 3 to about 5 carbon atoms,such as, for example: acrylic acid; cinnamic acid; methacrylic acid;crotonic acid; itaconic acid; maleic acid; and fumaric acid.

The resin is used most conveniently in the form of a latex which can beprepared according to available techniques.

A particularly preferred latex contains particles of resin prepared fromthe aforementioned monomers, which particles are chemically andphysically homogeneous. The resin particles of the preferred latex areprepared from styrene, butadiene, vinylidene chloride and methacrylicacid. In addition, the emulsifier content of the preferred latex isabout 1 to about 4% based on the resin solids and comprises at least 90wt. %, most preferably 100 wt. % of an anionic emulsifier such as asulfonate, for example, sodium dodecylbenzene sulfonate, or asulfosuccinate, for example, sodium oleoyl isopropanolamidesulfosuccinate, or a mixture thereof.

Although the coating composition can be contacted with the metallicsurface in a variety of ways, it is believed that the most widely usedmethod of contact will comprise immersing the metallic surface in thecoating composition at room temperature. As mentioned above, the longerthe metallic surface is immersed in the coating composition, the greaterthe buildup in coating thickness. It is believed that for mostapplications, desired coating thicknesses can be obtained by immersingthe metallic surface in the composition for a period of time within therange of about 30 seconds to about 3 minutes. However, it should beunderstood that longer or shorter periods of time can be used.

Agitating the composition aids in maintaining it uniform. Also,agitation of the composition is effective in improving the uniformity ofthe coatings formed.

Water rinsing the coated surface after it has been withdrawn from thecomposition, and before significant drying takes place is effective inremoving therefrom residuals such as acid and other ingredients of thebath that adhere to the coated surface. If such residuals are allowed toremain on the coated surface, they may change or adversely affect thequality of the coating. For a specific application, a determination canbe made as to whether the residuals cause adverse effects which are nottolerable. If they do, they should be removed, for example, by waterrinsing with tap or deionized water. If they do not, this step ofremoving them can be avoided.

Following any water rinse step that might be employed or after thecoated surface is withdrawn from the composition, it is contacted withthe aqueous Cr-containing composition of this invention (described indetail below) and then dried. Fusion of the resinous coating renders itcontinuous, thereby improving its resistance to corrosion and adherenceto the underlying metallic surface.

The conditions under which the drying and/or fusion operation is carriedout depend somewhat upon the type of resin employed. In general, heatwill be required to fuse the resin. The corrosion resistant propertiesof coatings fused at elevated temperature have been observed to bebetter than coatings which have been air dried. However, there areapplications where air dried coatings can be used satisfactorily. Thefusion of the coating should be carried out below temperatures whichcause the resinous coating to degrade. Exemplary conditions used infusing coatings produced according to the present invention aretemperatures within the range of about 100° to about 200° C. for periodsof time with in the range of about 10 to about 30 minutes, depending onthe mass of the coated part. Baking the coating for a period of timeuntil the metallic surface has reached the temperature of the heatedenvironment has been used effectively.

The improvement of this invention comprises contacting the wet oruncured autodeposited coating with an aqueous Cr-containing rinsesolution which also contains dispersed particles of a resin, which isdifferent from the autodeposited resin, and which has a coefficient offriction which is lower than that of the autodeposited resin. The rinsesolutions of this invention can be produced by dispersing the desiredresin, in latex form, in any of the known aqueous Cr-containingsolutions used in the prior art to treat autodeposited coatings.

The Cr-containing solution in which the resin is dispersed is typicallyan aqueous solution of hexavalent chromium or a mixture of hexavalentchromium and reduced forms of chromium.

The Cr-containing solutions containing hexavalent chromium only aretypically aqueous solutions of water or acid-soluble chromate ordichromate compounds or CrO₃. Useful compounds for this purpose includesodium dichromate, potassium dichromate, sodium chromate, potassiumchromate, and lithium chromate.

The Cr-containing solutions containing hexavalent chromium and reducedforms of chromium are described in U.S. Pat. Nos. 3,063,877 and3,795,546, the disclosures of which are incorporated herein byreference. Such solutions can be prepared from an aqueous concentratethat is made by treating an aqueous solution of chromic acid withformaldehyde to reduce a portion of the hexavalent chromium.

In terms of imparting a maximum degree of slip at the surface of theautodeposited coating, it is preferred that the aqueous Cr-containingsolution employed be one which contains only hexavalent forms ofchromium. Most preferably, the source of Cr in the solution is sodiumdichromate due to its excellent ability to enhance corrosion resistance.

The primary function of the Cr in the treating solution of thisinvention is the improvement of properties, such as corrosionresistance, of the autodeposited coating. Any amount of chromium in thesolutions will improve the properties of the applied coating. It ispreferred, however, that the total chromium concentration, includinghexavalent and reduced forms of chromium, if present, be at least about0.1 g/l and preferably at least 0.25 g/l in the treating solutions ofthis invention. The upper concentration limit of total chromium willgenerally be dictated by economic considerations in that incrementalincreases in the Cr concentrations do not produce correspondingimprovements in the coatings. Although the Cr can be present in amountsapproaching the solubility limit, there generally will be littleadvantage in using solutions containing more than about 5 grams of Crper liter.

The resin particles which are dispersed in the aqueous Cr-containingsolution are particles of any resin which has a coefficient of frictionlower than that of the autodeposited resin being treated. By this ismeant that the coefficient of friction between a steel surface and acontinuous surface (i.e., a fused surface) of the resin which isdispersed in the Cr-containing solution is less than the coefficient offriction between a steel surface and a continuous surface of the resinwhich has been autodeposited on the metallic surface. (See, e.g.,Handbook of Chemistry and Physics, 56th. Ed., p. F21 for coefficients offriction of various resins). Preferred resins for use in the aqueousCr-containing solution are polyethylene, polytetrafluoroethylene ormixtures thereof, due to their low coefficients of friction. Suitablepolytetrafluoroethylene and polyethylene latices which can be dispersedin aqueous Cr-containing solutions to produce the treating bath used inthis invention are commercially available, for example, under the tradenames Teflon 30 (polytetrafluoroethylene) and Poly Em 20 (polyethylene).

Any amount of the resin dispersed in the Cr-containing solution willalter the surface slip of the autodeposited coating which is treatedwith the solution. In terms of imparting a significant degree of surfaceslip to the autodeposited coating, it is preferred to employ the resinat a concentration of at least about 2.5 g/l, based on grams of resinsolids per liter of Cr-containing solution. There is no strict upperlimit on the amount of resin which can be dispersed in the Cr-containingsolution, however, no particular advantage is gained by employing theresin in the Cr-containing solution at a concentration greater thanabout 10 g/l.

The aqueous Cr-containing solution in which the resin is dispersed isacidic. The pH of the solution is normally from about 3.0 to 5.0.

There is the further proviso that the resin which is dispersed in theaqueous Cr-containing solution form a stable dispersion therein. This isnormally achieved by having a stabilizing amount of a surfactant on thesurfaces of the dispersed resin particles. It is preferred to use anonionic or an anionic surfactant. The concentration of surfactant whichis necessary to form a stable dispersion will vary, inter alia, with thepH of the solution, the concentration of resin dispersed in thesolution, and the particular type of resin and surfactant used. Theskilled worker will be able to determine an efficacious concentration ofsurfactant without undue experimentation. Typically, we have found thata concentration of from about 5 to 10 weight percent surfactant, basedon the weight of dispersed resin, has been sufficient to maintaindispersion stability.

The aqueous Cr-containing solution having the dispersed resin therein isapplied to the audodeposited coating on the metallic surface by anyconvenient means such as by immersing it in a bath consisting of theCr-containing solution and dispersed resin. The bath is not critical andthe temperature can vary from room temperature to about 175° F.

In order to maintain the effectiveness of the treating bath, it isnecessary to periodically replenish the dispersed resin in theCr-containing solution.

The examples which follow are intended to further illustrate theinvention described herein and are not intended to unduly limit thescope of the invention. Unless otherwise indicated, all parts andpercents are by weight.

The autodepositing coating composition used in the examples was preparedby combining the ingredients indicated below:

    ______________________________________                                        Ingredients            Amounts                                                ______________________________________                                        Latex containing about 54% solids                                                                    180      g.                                            Ferric Fluoride        3        g.                                            Hydrofluoric Acid      2.3      g.                                            Black pigment dispersion                                                                             5        g.                                            Water                  to 1,000 ml.                                           ______________________________________                                    

The resin of the latex used in the above composition comprised about 62%styrene, about 30% butadiene, about 5% vinylidene chloride and about 3%methacrylic acid. A film formed from the resin is soluble in refluxingchlorobenzene to the extent of about 13%. That the resin is crosslinkedis indicated by its insolubility in Soxhlet extraction withchlorobenzene. The water soluble content of the latex is about 2% basedon the weight of dried resin, with the water soluble content comprisingabout 10% sodium phosphate about 13% sodium oleoyl isopropanolamidesulfosuccinate and about 75% sodium dodecylbenzene sulfonate, the firstmentioned ingredient being a buffering agent used in preparing thelatex, and the last 2 mentioned ingredients being emulsifiers. The pH ofthe latex was about 7.8 and the surface tension thereof about 45-50dynes/cm. The average particle size of the resin was about 2,000 A.

The black pigment dispersion used in the above composition is an aqueousdispersion having a total solids content of about 36%. Carbon blackcomprises about 30% of the dispersion. It has a pH of about 10-11.5 anda specific gravity of about 1.17. The dispersion contains a nonionicdispersing agent for the solids, and is sold under the trademarkAquablak 115.

Unless stated otherwise, the metallic surfaces coated in the examplesbelow are unpolished cold rolled steel panels (Q-panels) 3"×4". Allmetallic surfaces were cleaned with a conventional alkali cleaner andrinsed with water prior to being coated.

The salt spray test used in the examples was in accordance with ASTMB-117 with the coating being scribed.

EXAMPLE 1

A series of panels were coated by immersing them in the autodepositioncoating bath for approximately 90 sec. The coated panels, identified asA-F below were then removed from the autodeposition coating bath andeach immersed in a different aqueous Cr-containing solution. In eachinstance the aqueous Cr-containing solution was prepared by adding apolyethylene latex to an aqueous solution containing 9 g./l. of Na₂ Cr₂O₇.2H₂ O, with the exception of the solution used to treat panel A. Thislatter solution was a control solution which contained no polyethylene.

The polyethylene latex which was added to the Cr-containing solutionswas an anionically dispersed latex having a 40% solids content ofpolyethylene which had a molecular weight of 15,000. The concentrationof polyethylene latex added to the aqueous Cr-containing solution usedto treat each of the panels is indicated in the Table below. Each of thepanels were immersed in the aqueous Cr-containing solutions for about 30seconds. The panels were then placed in an oven at 160° C. for 15minutes to fuse the coatings.

Each of the coated panels thus produced was tested for surface slip inthe following manner. A paper clip was placed at one end of the panel onthe coated surface. The end of the panel having the paper clip on thesurface was gradually raised to incline the panel. The height to whichthe panel had to be raised to allow the paper clip to slide down theinclined panel was recorded. The experiment was repeated three times foreach panel. The values given in the Table below represent the sum of theheights for the three repeated tests.

Additionally, each coated panel was tested for salt spray corrosionresistance for a period of 168 hours exposure and the results arereported in the Table below.

                  TABLE                                                           ______________________________________                                              Polyethylene latex added                                                                        Surface slip,                                                                            Salt Spray                                       to Cr Solutions   height of  Corrosion                                  Panel ml./l.            panel in cm.                                                                             Resistance*                                ______________________________________                                        A     0                 15.5       0-1/64                                     B     5                 11.9       0-1/32                                     C     10                10.1       0-1/32                                     D     25                 9.9       0-1/64                                     E     50                10.0       0-1/32                                     F     100               12.2       0-1/64                                     ______________________________________                                         *Measured as distance from scribe line of coating failure in inches.     

EXAMPLE 2

A steel panel was coated with an autodeposited coating in a mannersimilar to the panels of Example 1. The coated panel was then removedfrom the autodeposition bath and immersed in the following aqueousCr-containing solution:

    ______________________________________                                        Na.sub.2 Cr.sub.2 O.sub.7.2H.sub.2 O                                                                9        g.                                             Polyethylene latex*   25       ml.                                            Polytetrafluoroethylene latex**                                                                     1        ml.                                            Deionized water       to 1,000 ml.                                            ______________________________________                                         *Same as used in Example 1.                                                   **Nonionically dispersed, 60% solids.                                    

The coating was fused on the panel and the coated panel was tested forsurface slip in a manner similar to the coatings of Example 1. In threetest, the average height to which the panel needed to be raised to allowthe paper clip to slide off was 3.0 cm.

What is claimed is:
 1. A metallic surface having thereon a resin coatingcomprising the fusion product of autodeposited resin particles and fusedand concentrated in the surface layer thereof resin particles, which aredifferent than said autodeposited resin particles in having acoefficient of friction lower than that of the resin of saidautodeposited resin particles, the amount of said different resinparticles in the resin coating being such that said resin coating has ahigher degree of surface slip than a resin coating formed from saidautodeposited resin particles.
 2. A surface according to claim 1 whereinsaid fusion product includes chromium in an amount effective to improvethe corrosion-resistant properties of said resin coating.
 3. A surfaceaccording to claim 1 or 2 wherein said different resin particlescomprise polyethylene.
 4. A surface according to claims 1 or 2 whereinsaid different resin particles comprise polytetrafluoroethylene.
 5. Asurface according to claims 1 or 2 wherein said different resinparticles comprise polyethylene and polytetrafluoroethylene.
 6. Ametallic surface having thereon a resin coating comprising the fusionproduct of autodeposited resin particles, said particles prepared bycopolymerizing styrene, butadiene, vinylidene chloride, and methacrylicacid, and fused and concentrated in the surface layer thereof resinparticles which are different than said autodeposited resin particles inhaving a coefficient of friction lower than that of the resin of saidautodeposited resin particles, the amount of said different resinparticles in the resin coating being such that said resin coating has ahigher degree of surface slip than a resin coating formed from saidautodeposited resin particles.
 7. A surface according to claim 6 whereinsaid different resin particles comprise polyethylene.
 8. A surfaceaccording to claim 6 wherein said different resin particles comprisepolytetrafluoroethylene.
 9. A surface according to claim 6 wherein saiddifferent resin particles comprise polyethylene andpolytetrafluoroethylene.
 10. A metallic surface having thereon a resincoating comprising fused autodeposited resin particles and, fused andconcentrated in the surface layer thereof, resin particles which aredifferent than said autodeposited resin particles in having acoefficient of friction lower than that of the resin of saidautodeposited resin particles, the amount of said different resinparticles in the resin coating being such that said resin coating has ahigher degree of surface slip than a resin coating formed from saidautodeposited resin particles and has a higher degree of corrosionresistance than a coating comprising autodeposited resin particles of aresin having an inherently high degree of surface slip.